Passive dehumidifier

[Jacob]

You start to appreciate how well a dehumidifier works when it stops.
This is the water collecting channel as fixed to all the single glazed windows in our chapel conversion (still work in progress).
The pipe got blocked and this one filled up with water running out all over the place. There was a lot of it!
I've cleaned it up and it now drains well but not quite enough fall on it to clear completely - needs a bit more fettling, which I'll leave until next summer when it's dry.

The point is - it's amazingly effective and lots of water gets taken out, as soon as the temp falls enough. This means zero condensation on anything in the room as all will be warmer than the window glass.
One day I might attach a bottle to the outside to measure it.

 

[bugbear]

Since the items in the room are at the same temperature as the air in the room, they wouldn't attract condensation anyway.

The glass gets condensation because it's colder than the air in the room.

So, assuming your room thermostat is working, the room air temperature will remain nigh-constant, but in cold weather, the humidity (and relative humidity) would go down, due to the increased condensation onto the glass.

The little pipe does at least get rid of the water.

BugBear

 

[Jacob]

Since the items in the room are at the same temperature as the air in the room, they wouldn't attract condensation anyway.
The glass gets condensation because it's colder than the air in the room.

A common problem - particularly with double glazing, is that you get condensation on other things - usually the walls if uninsulated (black mould etc) when the glass is not the coldest surface.

So, assuming your room thermostat is working, the room air temperature will remain nigh-constant, but in cold weather, the humidity (and relative humidity) would go down, due to the increased condensation onto the glass.

The little pipe does at least get rid of the water.

BugBear

Er - what are you trying to say?

The point is - the glass acts as dehumidifier to the air in the room as it is the coldest surface and water condenses and drains off. It works almost exactly the same as a powered dehumidifier.
n.b. relative humidity goes up with falling temperatures - reaches 100% at the surface of the glass (or in the powered dehumidifier) when it is cold enough.

 

[bugbear]

So, assuming your room thermostat is working, the room air temperature will remain nigh-constant,

n.b. relative humidity goes up with falling temperatures

True, but irrelevant.

BugBear

 

[Jacob]

So, assuming your room thermostat is working, the room air temperature will remain nigh-constant,

n.b. relative humidity goes up with falling temperatures

True, but irrelevant.

BugBear

Irrelevant to what?
NB it's the driving principle behind dehumidifying by cooling.

 

[MikeG.]

.......The point is - the glass acts as dehumidifier to the air in the room as it is the coldest surface and water condenses and drains off. It works almost exactly the same as a powered dehumidifier.
n.b. relative humidity goes up with falling temperatures - reaches 100% at the surface of the glass (or in the powered dehumidifier) when it is cold enough.

This is all true, but it misses one of the critical factors in a dehumidifier: drawing the air to the cooling surface. Dehumdifiers have an in-built fan for this purpose, so can deal with much greater volumes of air than a simple cold surface can.

 

[MikeG.]

So, assuming your room thermostat is working, the room air temperature will remain nigh-constant,

n.b. relative humidity goes up with falling temperatures

True, but irrelevant.

BugBear

No, it's the most important factor in the whole process. The higher the relative humidity, the greater the propensity for the air to release it's moisture content. The cold surface of the dehumidifier locally raises the relative humidity to 100%, hence the condensation. However, cooler rooms dehumidify less well than might be expected because the temperature differential between the cold surface in the dehumidifier and the air is less than in a warm room.

 

[Jacob]

.......The point is - the glass acts as dehumidifier to the air in the room as it is the coldest surface and water condenses and drains off. It works almost exactly the same as a powered dehumidifier.
n.b. relative humidity goes up with falling temperatures - reaches 100% at the surface of the glass (or in the powered dehumidifier) when it is cold enough.

This is all true, but it misses one of the critical factors in a dehumidifier: drawing the air to the cooling surface. Dehumdifiers have an in-built fan for this purpose, so can deal with much greater volumes of air than a simple cold surface can.

The air circulates as it cools. Stand by a window on a cold day and you feel a cold down draught - warmer air is replacing it from the top. Yes a machine with a fan will circulate it faster.

 

[bugbear]

No, it's the most important factor in the whole process. The higher the relative humidity, the greater the propensity for the air to release it's moisture content. The cold surface of the dehumidifier locally raises the relative humidity to 100%, hence the condensation. However, cooler rooms dehumidify less well than might be expected because the temperature differential between the cold surface in the dehumidifier and the air is less than in a warm room.

Oh, hell yeah, it explains why you get condensation on cold glass.

The point is, in a room at a stable and uniform temperature (AKA "working heating system"), you don't have a change in temperature, thus no change in relative humidity in the room, thus no condensation problem to "solve" by pumping your heat out the window using the glass as a heat exchanger.

BugBear

 

[Jacob]

No, it's the most important factor in the whole process. The higher the relative humidity, the greater the propensity for the air to release it's moisture content. The cold surface of the dehumidifier locally raises the relative humidity to 100%, hence the condensation. However, cooler rooms dehumidify less well than might be expected because the temperature differential between the cold surface in the dehumidifier and the air is less than in a warm room.

Oh, hell yeah, it explains why you get condensation on cold glass.

The point is, in a room at a stable and uniform temperature (AKA "working heating system"), you don't have a change in temperature, thus no change in relative humidity in the room, thus no condensation problem to "solve" by pumping your heat out the window using the glass as a heat exchanger.

BugBear

No rooms have stable and uniform temperatures. People in the room increase humidity (breath). The cold surface of a window (or any other temperature differentials hot or cold) will set the air in motion. The relative humidity increases as the air circulates past the colder surfaces and vice versa as it passes the hotter surfaces.

 

[Jacob]

I didn't realise this was going to be a controversial thread! Not had to mention sharpening once!
In fact it's well known trad technology. Our chapel had drain channels cut into the stone cills with short lengths of lead pipe as the drain - just a 1" length visible outside under the window. The channels are still there but now boxed in and replaced by timber as per photo.
Quite common - particularly in large public buildings, either in the masonry or built up in timber as part of the cill.
Sash windows are very effective de-humidifiers too - water lost through the meeting rail gaps and at the cill. In kitchens/bathrooms etc you may find them with an adapted staff bead at the bottom, working as a drip catcher.

 

[bugbear]

I didn't realise this was going to be a controversial thread! Not had to mention sharpening once!
In fact it's well known trad technology. Our chapel had drain channels cut into the stone cills with short lengths of lead pipe as the drain - just a 1" length visible outside under the window. The channels are still there but now boxed in and replaced by timber as per photo.
Quite common - particularly in large public buildings, either in the masonry or built up in timber as part of the cill.
Sash windows are very effective de-humidifiers too - water lost through the meeting rail gaps and at the cill. In kitchens/bathrooms etc you may find them with an adapted staff bead at the bottom, working as a drip catcher.

You've definitely convinced me that single glazed windows stream with water in cold weather to the extent that they need drainage provision.

I'm yet to be convinced that this is a desirable. state of affairs.

I'm currently sitting in an office at 70°F with the outside temp at 2°C, and the window is perfectly condensation free (and thus I can see through it) and needs no drain.

It is (of course) double glazed.

BugBear

 

[John Brown]

"I'm currently sitting in an office at 70°F with the outside temp at 2°C"

Interesting. Would I be right in guessing that you work in the American Embassy in London?

 

[Jacob]

I didn't realise this was going to be a controversial thread! Not had to mention sharpening once!
In fact it's well known trad technology. Our chapel had drain channels cut into the stone cills with short lengths of lead pipe as the drain - just a 1" length visible outside under the window. The channels are still there but now boxed in and replaced by timber as per photo.
Quite common - particularly in large public buildings, either in the masonry or built up in timber as part of the cill.
Sash windows are very effective de-humidifiers too - water lost through the meeting rail gaps and at the cill. In kitchens/bathrooms etc you may find them with an adapted staff bead at the bottom, working as a drip catcher.

You've definitely convinced me that single glazed windows stream with water in cold weather to the extent that they need drainage provision.

I'm yet to be convinced that this is a desirable. state of affairs.

I'm currently sitting in an office at 70°F with the outside temp at 2°C, and the window is perfectly condensation free (and thus I can see through it) and needs no drain.

It is (of course) double glazed.

BugBear

You've probably got good ventilation and/or air conditioning or you would be getting condensation on your window, or the walls if external and not insulated. Depending on ambient conditions - very cold outside means dry air.

 

[MikeG.]

......... Depending on ambient conditions - very cold outside means dry air.

The relative humidity of the air outside will have no impact whatever on the internal condensation rates on a window, no matter what the walls and window are made of..........so long as the outside air isn't infiltrating.

My office (=house) is at roughly the same temperature (20.5C) and has never, ever had any condensation anywhere. We have a really interesting phenomena of having condensation on the outside of our windows on cold mornings. There isn't enough heat getting through them to warm the outer pane (triple glazing) enough to prevent it being cold enough to fall locally below the dew point of the air.

 

[Jacob]

...
The relative humidity of the air outside will have no impact whatever on the internal condensation rates on a window, no matter what the walls and window are made of..........so long as the outside air isn't infiltrating.......

If outside air isn't infiltrating one way or another - you all die of suffocation!
http://www.arca53.dsl.pipex.com/index_files/vent2.htm

 

[MikeG.]

You'll agree, though, that it is the internal relative humidity level which dictates internal condensation, rather than the external humidity level? Assume a low temperature externally, and a warm one internally.

 

[Jacob]

You'll agree, though, that it is the internal relative humidity level which dictates internal condensation, rather than the external humidity level? Assume a low temperature externally, and a warm one internally.

All depends. You can get changes of the weather where a cold night is follows by a warm and humid morning and suddenly all those chaps in sheds (where you get a rapid air exchange) find condensation all over their machines.
On the other hand internal activities (people breathing, cooking, washing) generate high humidity.
All internal air was external only a short while before, and brings with it the same absolute humidity and a matching dewpoint (relative humidity).

 

[MikeG.]

No, no, Jacob. It was your claim that:

....... very cold outside means dry air.

Whilst this is true, it isn't relevant. I am trying to understand how you think external relative humidity affects the dew point internally. You aren't explaining yourself clearly.

 

[Jacob]

No, no, Jacob. It was your claim that:

....... very cold outside means dry air.

Whilst this is true, it isn't relevant. I am trying to understand how you think external relative humidity affects the dew point internally. You aren't explaining yourself clearly.

No its the external absolute humidity i..e the actual amount of water in the air, relevant because it soon becomes internal air with the same absolute humidity, but change of relative humidity depending on change of temperature.
Cold air holds less water vapour and hence is often drier = lower absolute humidity